Electron discharge devices



Aug. 9, 1955 H. KLEMPERER 2,715,133

ELECTRON DISCHARGE DEVICES Filed Nov. 25, 1947 2 Sheets-Sheet l M U0 VMflaw Qw h Q m w H III/III [III II A Train 15') g- 9, 1955 H. KLEMPERER2,715,183

ELECTRON DISCHARGE DEVICES Filed NOV. 25, 1947 2 Sheets-Sheet 2 SWEEPVOLTAGE '1- ///Pl/T b 576ml 2 /0UTPUT\ V/DEO AMPLIFIER 5 sca PE 25 xiii/0 m 9 nraernoN DISCHARGE DEVICES Hans Klemperer, Belmont, Mass.,assignor to Raytheon Manufacturing Company, Newton, Mass., 21corporation of Delaware Application November 25, B47, Serial No. 7 87,873

7 Claims. (Cl. 250-27) This invention relates to electron dischargedevices, and more particularly to the type thereof known as storagetubes, in which an electron beam is used to place a charge on anon-conducting electrode disposed therein.

In a storage tube of the aforesaid type which is adapted to be used in aradar system, a signal such as a radar echo is stored in the form of anelectrical charge distribution on the surface of the non-conductingelectrode or storage plate. In moving target indicator (MTI)applications, an individual complete trace over the storage surface (onerecording) is compared with the next succeeding trace, and an output isproduced only when these two compared traces do not coincide with eachother. As long as there are no moving targets in the area. beingscanned, successive traces will be exactly the same and there will be nooutput signal. However, if moving targets are present, successive traceswill not be exactly the same and an output signal will be produced.

An object of this invention is to provide an arrangernent for biasingthe storage surface of a storage tube to a potential level which issubstantially different from that placed on it by the recording beam,thereby enabling an increased output signal to be obtained. Theforegoing and other objects of the invention will be best understoodfrom the following description of an exemplification thereof, referencebeing had to the accompanying drawings, wherein:

Fig. l is a cross-sectional view of an illustrative embodiment of thepresent invention;

Fig. 2 is a schematic representation of an embodiment of the presentinvention; and

Fig. 3 is a diagram of an equivalent circuit of the storage tube of thepresent invention.

Referring now more particularly to Fig. 1, the numeral 1 indicates anevacuated vessel usually associated with electron beam projection typeelectron discharge devices. Within said vessel there is disposed anelectron gun 2 (shown in broken outline), a pair of horizontallydisposeddeflecting plates 3 and a pair of vertically-disposed defiecting plates4. The electron gun 2 is located within the narrower portion 5 of thevessel 1, the method of projection of the electron beam therefrom beingquite familiar to those versed in the art of cathode-ray oscillography.

Disposed within said vessel in the larger portion 6 thereof is aplurality of planar parallel electrodes 7., 8 and 9, and extending fromthe narrower portion 5 to a position covering substantially half of thelarger portion 6 is an electrically-conductive coating 10. This coatingis suitably connected to apart of the electron gun structure 2 in amanner well known to those skilled in this particular art and serves toelectrically shield the electron beam represented by the broken lineoutline 11. The planes of electrodes 7-9 lie at substantially rightangles to the axis of beam 11, as shown.

The electrode 7, which is preferably constructed of nickel wire of verysmall diameter in the form of a 100- mesh screen, is disposedperpendicularly to said electron atent beam 11, said electron beampassing through said screen electrode 7 and impinging on the electrode3. This electrode is composed of a non-conducting or insulating materialsuch as glass. The electrode 9 is made of electrically-conductivematerial and is in intimate contact with the rear surface of electrode8, the electrode 9 preferably consisting of a conductive coating on theback of the electrode 8.

The electrode 7 is suitably supported by an electricallyconductiveannular member 12 and said annular member is in turn supported withinthe envelope 1 by a current-carrying conductor 13 suitably fused to saidenvelope and welded or brazed to the annular member 12, said conductor13 being terminated in an electricallyconductive cap 14 suitablyfastened to said envelope and conductor 13.

The electrodes 8 and 9 are supported by a second annular member 15 whichin turn is suitably attached and supported, as by welding or brazing, toan electrical conductor 16. Said last-named conductor passes through theenvelope 1 and is fused thereto so that said envelope serves to supportsaid annular member 15.

In order to maintain the electrodes 7 and 8 a predetermined distanceapart, a pair of spacing members 17 and 18 is respectively attached, asby Welding or other suitable means, to the annular member 12 and theconductor 16, the other ends of said spacing members being embedded inan electrically-insulating bead 19 which may be composed of glass orother electricallyinsulating material. The conductor 16 is terminated inan electrically-conductive cap 20 suitably attached to said conductor 16and the envelope 1.

Fig. 2 illustrates schematically the use of a storage tube, such as thatpreviously described, in connection with the visual reproduction of asignal due to the presence of a moving target in a given area scanned bya pulse-echo objectdetecting or radar system. In other words, thisfigure represents the use of such a storage tube in a so-called MTIsystem.

Cathode 21 of electron gun 2 is connected to the negative .end of asuitable source 22 of direct voltage, for example, a battery, thepositive end of which is connected to ground, so that the said cathodehas a high negative potential with respect to ground. The potential ofbattery 22 is quite high, on the order of 1600 volts, for example.Cathode 2.1, it will be understood, produces the necessary electronpopulation when suitably energized.

Accelerating anode 23 of the electron gun is connected to anintermediate point on battery 22 to provide the necessary bias thereon.In Fig. 2, for purposes of simplicity, the necessary focusing andcontrol electrodes included in gun 2 are not shown, nor are theirpotential sources; however, it is to be understood that such electrodesare included in gun 2 in a manner familiar to those versed in the artpertaining to this type of electron discharge device. Although in Fig. 2several sources of potential are shown in the form of batteries, theusual practice is to provide these potentials from a source ofsuitably-rectified alternating current.

Connected to one of the vertical deflecting plates 4 is a source of timesweep voltage 24. The circuit for generating said sweep voltage may beany of the many well-known circuits and is indicated on the drawings inblock form to simplify the illustration. One of the output terminals ofsource 24 is grounded, as is one of the plates 4, so that the electronbeam is deflected horizontally 'by the voltage supplied from source 24.Although it is not shown, it is to be understood that, in a radarsystem, the source 24 would be connected to the source of radartransmitter pulses to be triggered thereby at the time of eachtransmitted pulse.

Horizontal deflecting plates 3 are arranged to deflect the electron beamfrom gun 2 in a vertical direction. These plates are connected toreceive the output from an input signal source 25, such as a radarreceiver, which is shown in block form because such a receiver isconventional and is familiar to those skilled in the radar art. Theoutput of receiver is a series of pulses corresponding to thosereflected from reflecting objects in space within the field of search ofthe radar equipment.

The above-recited connections to the electron gun 2 and to the two pairsof deflecting plates are like those utilized in a so-called Type A radarindicator, in which the echo pulses cause upward deflections to occuralong the sweep trace on storage surface 8 at distances from thetransmitted pulse deflection proportional to the range of the target,the height of such deflections corresponding to the received signalintensity. Thus, the electron beam is swept across the storage surface 8in a repeated trace pattern and the radar receiver signal is applied insuch a manner as to deflect the beam in a direction perpendicular to thedirection of sweep.

Coating 10 is connected to ground, as at 26.

Collector screen or grid 7 is connected through a resistor 27 to groundat 28, so that said screen is at ground potential. Opposite ends of loador output resistor 27 are connected to provide the input to an outputamplifier 29, a direct current blocking condenser 30 being provided inseries in one of the leads to amplifier 29. The amplified output ofamplifier 29 may be applied to an oscilloscope 31, as shown.

The repeller electrode or metallic plate 9, which, as shown in Fig. l,is in intimate contact with the storage plate 8 made of insulatingmaterial, is connected through a high resistance 32, on the order of onemegohm, for example, to the negative side of a direct voltage source 33,the positive side of which is grounded at 34. The potential of battery33 is preferably on the order of 500 to 1000 volts. By this connection,the repeller 9 and thereby the whole storage plate 8 are raised to ahigh negative potential with respect to ground.

Although the repeller 9 is illustrated as being biased by an externalsource 33, it is possible to provide such bias in other ways. Forexample, the bias may be provided internally of the storage tube bysubjecting the storage surface 8 to unconcentrated electron bombardmentfrom a source of electrons inside the tube.

The secondary emission ratio of an electron target, whether an insulatoror a conductor, depends on the voltage of the incident electrons. Thesecondary emission ratio of such a target may be defined as the ratio ofthe secondary emission current leaving the target to the primaryelectron current striking the target, and for each material there is acertain value V1 of primary electron voltage which gives for suchmaterial a secondary emission ratio of unity. When the voltage of theprimary electron beam is less than the value V1, the secondary emissionratio is less than unity and the number of electrons striking the targetexceeds the number leaving the target, while, when the voltage of suchbeam is greater than V1, the secondary emission ratio is greater thanunity and the number of electrons leaving the target exceeds the numberstriking the target.

For explaining the action of the tube, we will disregard the negativepotential of the repeller 9 for an instant. The electron beam is of highvoltage with respect to ground or with respect to plate 8 if said plateis at ground potential, this high voltage being on the order of 1600volts, for example. This voltage is substantially greater than theso-called critical voltage V1 defined above, which critical voltagegives a secondary emission ratio of unity for target 8. Therefore, thenumber of secondary electrons leaving the target 8, from the areasbombarded by the electron beam of gun 2, is in excess of the number ofprimary electrons striking said target in such areas. Since moresecondary electrons are leaving the target than are striking it, a netpositive charge or voltage tends to be produced on plate 8 in the areasbombarded by the electron beam from gun 2.

The electron beam current is made sufficiently high, with a given sweepor writing speed, to produce an equilibrium potential, or an equilbriumpotential of zero current, throughout the areas bombarded by the mainelectron beam. This equilibrium condition results from the followingaction. As the surface of plate 8 builds up a positive potential, due tothe secondary emission ratio being greater than unity, as describedabove, the positive charge on this plate tends to attract the secondaryelectrons produced back to the plate, since such electrons have a ratherslow velocity. The positive potential of surface 8 also tends toincrease the velocity of the primary electrons striking said surface, tothereby produce more secondary electrons, but this effect is greatlyovershadowed by the attracting or retarding effect of thepositively-charged plate on the secondary electrons, since the primaryelectrons are of very high velocity as compared to the velocity of thesecondary electrons, and it therefore requires a much greater voltagechange to produce an appreciable effect on the primary electrons than onthe secondary electrons. This retarding or attracting effect on thesecondary electrons increases as the voltage of plate 8 becomes morepositive, until a point is reached at which the number of secondaryelectrons which succeed in escaping the retarding or attracting voltageof plate 8, and which therefore leave said plate, equals the number ofprimary electrons striking the plate, giving a zero net current at thisequilibrium point.

It has been found that this equilibrium condition is reached when thepotential of plate 8 is on the order of two volts positive with respectto screen 7 or ground. Therefore, since this equilibrium condition isreached in a single trace or sweep of the main electron beam acrosssurface 8, a completed trace will leave the storage surface 8 coveredwith a line of discrete charges at equilibrium potential, or having apredetermined value of potential with respect to ground, on the order oftwo volts positive, for example.

By the above-described action, the beam, under these conditions,produces a trace which is slightly positive with respect to ground. Someof the secondary electrons liberated by the tracing or progressing beamare collected by the collecting screen 7 and some return to the storagesurface. Those which return to the storage surface ordinarily return tothose parts of the surface which were left positively charged, causing aslight negative charge bordering the positive trace and reducing thepositive charge of that portion of the trace which the beam has justpassed. This obliterating action tends to destroy the trace and greatlyreduces the sensitivity of the storage tube, particularly since thepotential difference between trace and no trace under these conditionsis comparatively small.

By raising the repeller 9, and thereby the whole storage plate 8, to ahigh negative potential, the sensitivity of the tube is greatlyimproved.

The potential of the storage plate 8 and repeller 9 relative to thepotential of cathode 21 will determine the speed of the electronsstriking the storage surface 8. In the example given, the voltage of therepeller 9 may be 1,000 volts negative with respect to ground, while thevoltage of cathode 21 may be 1,600 volts negative with respect toground, so that a difference of potential of 600 volts exists betweencathode 21 and plate 8, cathode 21 being 600 volts negative with respectto plate 8. This voltage diiference is substantially greater than thesocalled critical voltage V1 defined above, which critical voltage givesa secondary emission ratio of unity for target 8; therefore, moresecondaries are leaving than arriving at the point of incidence of theelectron beam on the storage surface. Due to the negative voltage ofrepeller 9, secondaries will continue to leave the storage surface 8 ingreater numbers than they arrive until the potential of the storagesurface becomes positive with respect to ground, or screen 7, to asufiicient extent to establish equilibrium, which, as has been pointedout above, is on the order of two volts positive with respect to groundor screen 7. Since this is so, the trace will still be slightly positivewith respect to the potential of the collector 7, due to theabove-explained action, so that the charge of the trace changes from theoriginal unbombarded 1,000 volts negative with respect to ground to twovolts positive with respect to ground. The change in charge of the traceis therefore increased (by applying a sufficient negative potential torepeller 9) from a few volts to the order of 1,000 volts. The beam tracewill therefore produce a line on surface 8 along which the potential isuniform and predetermined, said potential being greatly dif erent fromthe potential of those areas not bombarded by the electron beam becausethe areas not so bombarded remain at the negative 1,000 volts potential(with respect to ground) applied to storage surface 8 by means ofrepeller 9 and battery 33.

Also, the secondary electrons have an increased tendency to stay awayfrom the storage surface 8 when a volta e is applied to repeller 9,since they are repelled by the hi h negative potential surrounding thetrace. Therefore, the secondaries do not return to the storage plate butare forced to the collector 7 or to the so-called second anode 19. As aresult, the deleting action caused by secondaries upon earlier parts ofthe trace is greatly reduced, resulting in a sharper positive trace.

With the repeller 9 energized, the difierence of potential between traceand no trace is very large, and also the obliterating or deleting actionof the secondary electrons is substantially eliminated. By applying arepeller potential the output signal is raised, roughly speaking, frommicrovolts to millivolts (maximum on the order of miliivolts), giving agreatly increased tube sensitivity. By the biasing of surface 8 to ahigh negative potential by repeller 9 and battery 33, I can obtain alarger output signal than if no biasing were used, since the potentialchange of record points on surface 8, produced by the electron beam inbringing such points from the biasing potential value to the smallpredetermined potential (with respect to ground) value or equilibriumvalue in the manner described, is much greater than that which wouldresult from bringing such points merely from zero potential to the samesmall predetermined equilibrium potential value with respect to ground.

The voltage across resistor 27 is used as the input signal to amplifier29, so that collecting screen 7 is capable of being utilized as a signalelectrode. Fig. 3 shows an equivalent circuit of the storage tube.Points 7, 8 and 9 designate the collector, storage surface, andrepeller, respectively, these points being connected by capacitances andresistances as shown.

C1 represents the repeller-collector grid capacitance and is on theorder of 50 micrornicrofarads, varying, of course, with the constructionof the tube. C2 and C3 are the capacitances between the bombardedelement on the storage surface 3 and the repeller and collector,respectively, and depend on the concentration of the cathoderay spot ofthe input electron beam 11 which strikes storage surface 8. C4 is thecapacitance between collector grid 7 and the coating or second anode 13.

The resistance R2 shunting C2 corresponds to the conductivity of thestorage plate 8 which is made of a material which is a very poorconductor, such as glass or mica, for example. The product R2 C2 iscalled the time constant of the storage tube and defines the memorytime; time constants for various storage plate materials may range froma fraction of a second to hours or even days, depending upon theparticular material.

The paths of the incident electron beam 11 and of the secondaryelectrons are represented by arrows; as indicated, some of the secondaryelectrons ejected from plate 8 return thereto, some proceed to collectorgrid 7, and some proceed to anode 10. The action of the secondaryelectrons upon plate 8 can be approximated by a series arrangement of abattery 36, a switch 35, and a variable resistance R3, the actioninsofar as anode 10 is concerned involving variable resistance RA inseries rather than R3. The closing of the switch 35 corresponds to theincidence of the electron beam 11. Since the .electron beam leaves apositive charge on plate 8, battery 36 is poled as indicated.

The external repeller resistance 32 is on the order of one megohm; theexternal collector resistance 27 is considerably smaller, on the orderof 50,000 ohms, to present an appropriate input impedance to the outputamplifier 29.

The repeller resistance 32 is critical; it should not be much smallerthan one megohm, since otherwise the output signal is reduced in size.The distance from the repeller 9 to the beam side of the storage surface8 has a large influence upon the amplitude of the output signal; tubeswith close spacing are more sensitive than tubes with a wider spacing.

The sign of the collector output signal voltage is positive. Referringto Fig. 3, the action of the electron beam 11 upon plate 8 with respectto the collector 7 (the closing of switch 35) is twofold. Firstly,liberated secondary electrons tend to induce negative signals when theystrike the collector '7. However, a large fraction of the total numberof secondaries liberated is lost through the holes in grid 7 to thesecond anode 10; in fact, the second anode current is ordinarily on theorder of ten times the collector current. Secondly, dielectricdisplacement, caused by the change of potential of plate 8 from a highnegative toward a positive potential by the beam 11 in the mannerexplained previously, is transferred through C2 and C1 in series, andalso through C3, to the collector '7. This dielectric displacementcurrent flows in the opposite direction to that produced by the movementof the secondary electrons which strike collector 7; this dielectricdisplacement current is larger than that produced by the movement of thesecondary electrons, so that the net collector signal or output voltageis positive.

A negative output signal indicates that the electron beam 11 has stayedon the bombarded spot (on plate 3) long after the change in surfacepotential is efiected. Displacement currents have subsided, under theseconditions, and the negative charge from arriving secondaries prevailsat the collector '7. The weak negative signal is therefore an indicationof improper adjustment of beam intensity; the output signal reverses toa weak negative signal when a very strong beam intensity is used.

On the other hand, a strong positive output signal requires a so-calledfree-swinging repeller 9, or one which has a very high repellerresistance 32 connected in series between the repeller and the potentialsource. The strong output signal is reduced with increased capacitanceto ground C of the repeller 9. if the positive transient, produced bythe action of the electron beam 11, is suppressed at the repeller bymeans of a considerably in creased C positive and negative impulses onthe collector 7 cancel out almost completely; low ground capacitance ofthe repeller 9 and its associated leads is thereelectron beam, insteadof being caused to swing across the surface of the storage plate, may bemerely caused to impinge thereon at certain predetermined points. Also,this repeller type storage tube can be utilized for general laboratorywork in cases where a discriminating memory action is required. Forexample, the tube could be used to obtain records of transientunforeseen electrical phenomena, such as lighting or arc-backs inrectifiers. In such applications, the storage tube, by its action asdescribed previously, would cancel out any periodically repeatinginformation, and Would supply a record only of anything unforeseen. Aphotograph could be taken of the oscilloscope pattern, in order toretain information regarding the unforeseen phenomena. Various othervariations will suggest themselves. It is accordingly desired that theappended claims be given a broad interpretation commensurate with thescope of this invention within the art.

What is claimed is:

1. An electron discharge device circuit comprising an electron gun forprojecting a beam of electrons; a potential storing target having apotential storing surface in the path of projection of said beam; anelectrode closely adja cent and capacity coupled to said surface; saidpotential storing surface being disposed between said electron gun andsaid electrode; a source of fixed voltage coupled to said electrode andsaid gun for applying a predetermined fixed voltage positive withrespect to said gun to said electrode; an electron permeable collectormeans closely adjacent and spaced from said target surface on the sidethereof facing said gun; and means directly electrically connected tosaid collector means including means for deriving an output signaltherefrom.

2. An electron discharge device circuit comprising an electron gun forprojecting a beam of electrons; a potential storing target in the pathof projection of said beam having a potential storing surface facingsaid electron gun and an opposed surface; an electrode closely adjacentand capacity coupled to said potential storing surface; an electronpermeable collector means closely adjacent and spaced from said storingtarget on the side thereof facing said gun; a source of fixed voltagecoupled to said electrode and said gun for applying a predeterminedfixed voltage positive with respect to said gun and negative withrespect to said collector means to said electrode; and means directlyelectrically connected to said collector means including means forderiving an output signal therefrom.

3. An electron discharge device circuit according to claim 2 furtherincluding means for repetitively sweeping said beam across said storingsurface of said target at a predetermined rate; and means responsive toan input signal for deflecting said beam in a direction perpendicular tothe direction of said sweep.

4. An electron discharge device circuit according to claim 2 furtherincluding means including said source for initially biasing said storingsurface negative with respect to said collector means; said surfacecharging toward a predetermined equilibrium potential value duringimpingement of said storing surface by said beam which is positive withrespect to said collector means.

5. An electron discharge device circuit according to claim 2 whereinsaid source is of such magnitude as to give a secondary emission ratiogreater than unity for said potential storing surface of said target sothat the portion of said surface impinged by said beam tends to chargetoward a predetermined equilibrium potential value which is positivewith respect to said collector means.

6. An electron discharge device circuit comprising an electron gun forprojecting a beam of electrons; a potential storing target in the pathof projection of said beam having a potential storing surface facingsaid electron gun and an opposed surface; an electrode closely adjacentand capacity coupled to said potential storing surface; an electronpermeable collector means closely adjacent and spaced from said storingtarget on the side thereof facing said gun; a source of fixed voltagecoupled to said electrode and said gun for applying a predeterminedfixed voltage positive with respect to said gun and negative withrespect to said collector means to said electrode; means directlyelectrically connected to said collector means including means forderiving an output signal therefrom; means for repetitively sweepingsaid beam across said storing surface of said target at a predeterminedrate; and means responsive to an input signal for deflecting said beamin a direction perpendicular to the direction of said sweep; meansincluding said source for initially biasing said storing surfacenegative with respect to said collector means; said surface chargingtoward a predetermined equilibrium potential value during impingement ofsaid storing surface by said beam which is positive with respect to saidcollector means.

7. An electron discharge device circuit comprising an electron gun forprojecting a beam of electrons; a potential storing target in the pathof projection of said beam having a potential storing surface facingsaid electron gun and an opposed surface; an electrode closely adjacentand capacity coupled to said potential storing surface; an electronpermeable collector means closely adjacent and spaced from said storingtarget on the side thereof facing said gun; a source of fixed voltagecoupled to said electrode and said gun for applying a predeterminedfixed voltage positive with respect to said gun and negative withrespect to said collector means to said electrode; means directlyelectrically connected to said collector means including means forderiving an output signal therefrom; means for repetitively sweepingsaid beam across said storing surface of said target at a predeterminedrate; and means responsive to an input signal for deflecting said beamin a direction perpendicular to the direction of said sweep; said sourcebeing of such magnitude as to give a secondary emission ratio greaterthan unity for said potential storing surface of said target so that theportion of said surface impinged by said beam tends to charge toward apredetermined equilibrium potential value which is positive with respectto said collector means.

References Cited in the file of this patent UNITED STATES PATENTS2,403,239 Rose .a July 2, 1946 2,437,173 Rutherford Mar. 2, 19482,454,410 Snyder Nov. 23, 1948 2,548,405 Snyder Apr. 10, 1951 2,618,762Snyder Nov. 18, 1952

